Global demand for light olefins such as propylene, butylene, and ethylene is rapidly increasing due to wide applications of their derivates in the field of consumer durables, packaging, automotive, construction, medical, etc. Conventional sources of propylene are steam cracking of light hydrocarbons and also fluid catalytic cracking (FCC). With the discovery of shale gas reserves, ethylene production has increased significantly by steam cracking of lighter feedstock while propylene or butylene is obtained as by-product. This has resulted in a gap between the supply and demand for propylene or butylene and thus, a huge increase in the prices of both. In such scenario, alternative routes for on-purpose propylene/butylene production such as propane/butane dehydrogenation, olefin metathesis and methanol to olefins (MTO) have become significant.
U.S. Pat. No. 8,563,793 describes an integrated process for recovery of propylene from the hydrocarbon stream consisting of propane and/or C4-components, and catalytic dehydrogenation of propane to propylene, wherein purified propane fraction from PRU and recycled hydrogen stream are sent to dehydrogenation section to produce propylene. Dehydrogenation of propane (or any alkane) is an endothermic reaction and is limited by its thermodynamic equilibrium, due to which higher temperatures (usually above 600° C.) are required to achieve economically feasible conversions. Such high temperatures cause thermal cracking of hydrocarbons lowering selectivity of desired unsaturated hydrocarbon propene (or corresponding alkene) and accelerate catalyst deactivation. In order to maintain lower partial pressures of alkanes, the system is diluted with hydrogen stream which results in limited conversion and alkene selectivity.
Alkane dehydrogenation in the presence of oxygen can be performed at lower temperature. Moreover, the dehydrogenation equilibrium can be shifted forward to obtain higher alkane conversions by the reaction of oxygen with the hydrogen co-product. Oxygen is believed to burn the coke off the catalyst and thereby keep the surface clean from coke deposition, leading to increased catalyst life. U.S. Pat. No. 4,788,371 A discloses an oxidative dehydrogenation (ODH) process, wherein the alkane feed along with oxygen and steam is introduced into the reactor containing noble metal catalyst. The effect of oxygen here is to combust hydrogen with minimal combustion of hydrocarbons.
Alkenes produced by ODH process are easily oxidized in the presence of oxygen, and thus, the selectivity of alkenes decline rapidly with increase in conversion of alkanes. Another drawback of using oxygen as a promoter in dehydrogenation reaction is that, it requires special handling and explosive protection apparatus for its safe operation in commercial scale, since oxygen can form explosive mixtures with hydrocarbons. Therefore, mild oxidant such as carbon dioxide is extensively used to improve the propane conversion in oxidative dehydrogenation reactions with minimum effect on the propylene selectivity. Utilization of carbon dioxide in industrial process also helps in the mitigation of CO2 from the atmosphere. CO2 not only suppresses the unwanted total oxidation products due to its lower oxidizing ability, but also improves product selectivity. In the presence of carbon dioxide, the propane dehydrogenation proceeds in oxidative pathway whose rate of the reaction is faster than the direct dehydrogenation reaction thereby producing higher yield of propylene (T. Shishido et al., Catal. Today, 2012, 185, 151-156).
Numerous catalyst formulations for propane dehydrogenation using carbon dioxide have been disclosed. For example, U.S. Pat. No. 7,094,942 B2 describes the process where the alkanes are contacted with Cr-based dehydrogenation catalyst in the presence of CO2 to produce corresponding alkenes at a temperature ranging from 400° C. to 700° C., a pressure ranging from 0.1 to 10 atm, wherein the alkane to CO2 molar ratio is about 1:0.0001 to 1:0.045.
U.S. Pat. No. 182,186 A1 discloses the process for dehydrogenation of propane to propylene using a silica chromium catalyst composition in the presence of CO2. Further, the silica Cr catalyst composition may include a promoter, such as, V, Ag, Ce, Zn, Zr, etc.
Reduction in emissions of greenhouse gases like carbon dioxide to the atmosphere is gaining momentum to combat the increasing global warming. Therefore, there exists a need for developing environmental friendly and cost-effective processes that utilize feed sources from existing refinery.